Thank you, MrAl, for detailed explanation.
I have just followed your explanation and indeed, D = 0 and it means that no conclusion can be drawn.
Do you mean that we has to graph the function and find its maximum visually? If so, I think we can't get the exact value and if the function is very complicated it is very hard to graph too.
Does this mean that the critical point can be maximum or minimum and therefore we can test it by substituting them into the function and check it?
This is what I am confused. Here we have a two-variable function and the maximum value is found by making a variable constant and the function is simplified to one variable. Thus, we can find the value of the variable for the function get maximum.
The next step is similar by exchange the role of two variable.
However, how can you know that this method will work?
PS.
I think the variables here is RL and XL not Xs and XL. Could you explain why?